12 OYSTERS AND DISEASE. 



As to the probable function of this muscle in the existing oyster, its action 

 clearly must be to draw the labial palps and the anterior end of the branchiae forwards 

 and outwards. When this action is imitated by means of forceps on the fresh oyster, 

 the result is seen to be that the funnel leading to the mouth is widened and opened, 

 and the " catchment area " by which the mouth is supplied becomes increased. We 

 give a diagram, in Fig. 14, showing the relations of the protractor muscles of the two 

 sides to the mouth and neighbouring parts, and we add in dotted lines the probable 

 course of the muscles before the foot disappeared in ancestral oysters. It seems to us 

 that when the muscle was no longer needed as a protractor pedis, it may readily 

 have come to serve its present useful purpose by the posterior half of the muscle 

 aborting, while the anterior half remained and acquired an attachment to the connective 

 tissue of the palp and the gill through which it must have formerly passed, and where 

 it is quite likely some of the fibres always ended. 



Our figure (PI. II., Fig. 14), which, although drawn as a diagram, is true in its 

 proportions, and was sketched directly from the living animal, shows clearly the con- 

 figuration of the labial palps when the two protractor muscles are slightly drawn upon. 

 The inner or posterior palps curl together in the middle line behind the mouth, while 

 the outer or anterior palps diverge widely. The anterior ends of both gills on each side 

 are received between the outer and inner palps of that side ; so that the only avenue 

 to the mouth, the passage on each side between the two palps, becomes much widened 

 at its posterior end, more funnel-shaped, and evidently better adapted for collecting 

 the food particles from the anterior ends of the gills, and passing them forward to 

 the mouth. 



The action of the oyster in opening its shell (i.e., divaricating the valves), 

 preparatory to feeding, separates the points of attachment of the two muscles, and so 

 of itself, even without any muscular contraction, causes the opening up of the food 

 avenues described above : the contraction of the muscles which doubtless takes place 

 will emphasize the same action.* 



* The technical term " fishing " used by oyster-growers and others connected with the trade, to indicate the 

 act of feeding in an oyster, is therefore so far a correct description, inasmuch as the action is comparable with the 

 spreading of a net. 



